1. Field of the Invention
The present invention relates to an exposure head for exposing an exposure surface at a recording medium or the like with a laser beam which is spatially modulated by a spatial modulation element in accordance with image data, and to an exposure apparatus equipped with this exposure head.
2. Description of the Related Art
Heretofore, various exposure apparatuses which employ spatial light modulation elements such as digital micromirror devices (DMD) and the like have been proposed for carrying out image exposure with light beams modulated in accordance with image data.
An example of a DMD is a mirror device in which numerous micromirrors, which alter angles of reflection surfaces thereof in accordance with control signals, are arranged in a two-dimensional manner on a semiconductor support of silicon or the like. An example of an exposure apparatus which utilizes such a DMD is, as shown in FIG. 27, structured by a light source 1 which irradiates a laser beam, a lens system 2 which collimates the laser beam irradiated from the light source 1, a DMD 3 which is disposed substantially at a focusing position of the lens system 2, and lens systems 4 and 6 which focus the laser beam that has been reflected at the DMD 3 onto an exposure surface 5. Although the DMD 3 is a reflection-type spatial modulation element, in FIG. 27, for ease of explanation, the laser beam is shown as being emitted to the exposure surface 5 side from the DMD 3 without being deflected.
In this exposure apparatus, the respective micromirrors of the DMD 3 are switched on and off by an unillustrated control apparatus, in accordance with control signals generated in accordance with image data or the like, and modulate (deflect) the laser beam. The exposure surface is exposed to two levels by the modulated laser beam. Here, the lens systems 4 and 6 are structured to serve as magnifying optical systems, and enlarge an exposure area at the exposure surface 5 relative to a surface portion of the DMD 3 at which the micromirrors are disposed.
In the exposure apparatus described above, generally, the micromirrors at the DMD and the exposure surface 5 are set to be mutually conjugative, reflected images from the micromirrors are focused on the exposure surface 5 by the lens systems 4 and 6, and these reflected images expose the exposure surface 5 in the form of beam spots.
However, when the size of the exposure area on the exposure surface 5 is magnified relative to the area of the surface portion of the DMD 3 by the lens systems 4 and 6, the area (spot diameter) of the beam spots on the exposure surface 5 is also magnified in accordance with the magnification rate. Consequently, an MTF (modulation transfer function) characteristic at the exposure surface 5 is reduced in accordance with the magnification rate of the exposure area.
Accordingly, in the exposure apparatus described above, as shown in FIG. 28, a plurality of microlenses 7 may be disposed between the lens system 6 and the exposure surface 5 in a one-to-one correspondence with the respective micromirrors of the DMD 3. The laser beams reflected by the micromirrors are contracted by the microlenses 7. Thus, the spot diameter at the exposure surface 5 is adjusted (reduced) to a desired size.
However, in the exposure apparatus described above, laser emission portions of the light source 1 have some area and are not generally expressed as point light sources. Therefore, as shown in FIG. 27, the laser beams reflected by the micromirrors of the DMD 3 have a certain spreading angle αs corresponding to the area of the laser output portion. As a result, in the exposure apparatus described above, in a case in which microlenses are utilized in order to shrink the spot diameter of the beam spots on the exposure surface 5, a portion of the laser beam that is reflected by a particular micromirror is incident at the microlenses 7 other than the one of the microlenses 7 that corresponds to the particular micromirror. If there is a lot of such stray light SL, the stray light SL passes through the microlenses 7 and forms a ghost image GI on the exposure surface 5. Hence, exposed portions which are noise portions are caused at the exposure surface 5.
Further, for the exposure apparatus described above, not utilizing the microlenses 7 but disposing apertures, which have an opening diameter corresponding to the required spot diameter of the beam spot, between the exposure surface 5 and the lens system 6 and shrinking the beam diameter at the exposure surface 5 with these apertures has been considered. However in a case in which such apertures are used, light amount losses due to the apertures increase in accordance with increases in a rate of contraction of the laser beams, and light usage efficiency is greatly reduced.